In the manufacture of carboxylic acids frequently aldehydes are produced as by-product impurities. This is especially true in the process by which olefins are oxidized with molecular oxygen to produce unsaturated acids where aldehydes are produced as an intermediate and these frequently are found in the end product acid. Thus, acrolein is found as a minor impurity which is difficult to remove completely by distillation from acrylic acid. Neither are such by-products removed completely by the extraction process normally used as part of the purification process.
Acrolein, and other carboxylic compounds, e.g. acetaldehyde, furfural, glyoxal and crotonaldehyde, are undesirable in the unsaturated acids when these are used for making polymers and plastics because they cause (1) a prolonged induction period in the polymerization reaction, (2) color in the resulting products made from these polymers, (3) crosslinking and (4) chain termination which results in lower molecular weight polymers.
Various methods are suggested in the art for removing these aldehyde impurities from the acids. For example, U.S. Pat. No. 3,725,208 discloses a method whereby sulfuric acid, hydrazine, phenylhydrazine, aniline, monoethanolamine, ethylene diamine or glycine are added to the acid prior to its distillation. Another patent, U.S. Pat. No. 3,893,895 claims reacting the carbonyl compounds with aliphatic or aromatic amines such as hydroxylamine, a monoalkyl amine, naphthylamine to chemically bind the carbonyl compounds and then recovering said carboxylic acid by distillation.
In such processes some of the amine reacts with the acid as well as the aldehydes present with consequent loss in yield of the acid.
It has now been discovered that some of the aldehyde impurities in acrylic acid selectively react with 1,2-glycols added to the acid and, upon subsequent distillation can be removed (since the product remains in the bottoms) with no loss of acrylic acid product.